1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to methods for manufacturing disk drives.
2. Description of the Prior Art
Hard disk drive storage devices have been used as a secondary storage device for computer systems for many years. They provide inexpensive, high capacity digital storage with the ability to quickly access data stored on the drive. A typical hard disk drive comprises a housing with one or more magnetic disks separated by spacers mounted on a motor driven spindle hub that in turn is supported on a base. Data is stored on the disks by changing the magnetization of small areas on the disks called domains. The domains are written and read by magnetic transducers mounted on the end of suspensions that are coupled to a rotary actuator positioned to one side of the disks. The rotary actuator moves the transducers radially over the surfaces of the disks. Data on the disks are written in substantially parallel concentric tracks, with information, called servo information designed to be detected by the transducers and used to control the radial position of the transducer so that it can move across tracks of data and follow a track of data.
It is crucial that the disk pack is balanced so that it does not cause significant vibration of the disk drive when the drive is rotated at high speed. A disk pack typically includes a spindle motor, at least one disk, a disk spacer between a pair of disks and a disk clamp. Typical disks rotate at between 5400 to 15,000 revolutions per minute (RPM) and higher. An out of balance condition in the rotating disk pack causes erratic speed variations with respect to the tracks and heads that results in read/write errors. Planar and axial vibrations of the disk surfaces can also contribute to head crashes, harming both the disk surface and the head. Disks that significantly vibrate when rotating also increase track misregistration, cause annoying acoustical emissions (noise) when the disk drive is mounted in a customer chassis, such as a computer chassis and a reduction in the life of spindle bearings. When such disk drives are used in an array of disk drives, the mechanical vibrations of the multiple drives can become synchronized, resulting in unacceptable vibration of the entire array. There is a need, therefore, to ensure that the disks are centered when clamping them to the spindle hub to avoid these problems.
To correct or improve such imbalances, among other methods of balancing disk packs, commonly assigned U.S. Pat. No. 5,824,898 (incorporated herein by reference) proposes to reduce the amplitude of rotational vibration of a disk pack in a disk drive by imparting acceleration to a support thereof to shift it as a function of measured rotational vibration. According to one embodiment described in this patent, as the motor rotates the hub and disk stack, a sensor detects vibrations of the base caused by the disk pack imbalance. A momentum transferring force is then applied to the base, causing the disk stack to shift relative to the base in a manner designed to reduce the imbalance and hence the induced vibrations. Multiple instances of momentum imparting force may be applied until the vibrations are within a predetermined value.
By shifting the entire disk stack in unison through the application of the momentum imparting force, the disks may become substantially and concentrically aligned relative to one other following the balancing. In addition, the inherent vibrations that cause the rotational imbalance in the motor and hub combination are offset by a substantially equal and opposite imbalance in the disk stack. Reducing the disk pack imbalance in this manner also beneficially reduces acoustical noise.
As suggested by FIG. 1, a number of disk drive manufacturing steps are carried out, including coupling the disk pack to the base. Thereafter, the disk pack imbalance of each drive is measured (for example, by rotating the disk pack and measuring the disk pack imbalance while the disks are rotated). Following the disk pack imbalance-measuring step, each of the measured disk drives conventionally undergoes a balancing step, followed by subsequent manufacturing steps, including servo writing, for example. Conventionally, each disk drive passes through a disk pack balancing station, irrespective of whether the measured imbalance is within an acceptable range or not. During the balancing process, the disk pack imbalance of each drive is iteratively measured, corrected and verified. Routing each of the disk drives through such a disk pack balancing station or stations and/or subjecting each drive to such balance reducing steps is costly in terms of manufacturing costs, but also in terms of throughput of the entire manufacturing assembly line. Indeed, it may take as much as 20 to 50 seconds per drive to measure the disk pack imbalance of the drive, reduce the measured imbalance and verify the resulting balance of the disk pack of the drive. Conventionally, one way to reduce the costs associated with such a balancing process was to route the drives through a number of balancing stations in parallel. However, this solution is less than optimal, as it merely functions to reduce the width of the bottleneck through which each drive must pass and geometrically increases the disk pack balancing costs, as several disk pack imbalance reducing machines must be purchased, installed, run and maintained.
What are needed, therefore, are methods of manufacturing disk drives that reduce the costs associated with reducing disk pack imbalance and that increase the manufacturing throughput.